Process for producing polyolefins

ABSTRACT

A process is provided to produce polyolefins having a multimodal molecular weight distribution at the molecular level by contacting at least one mono-1-olefin in a polymerization zone, under polymerization conditions, with dual site supported chromium polymerization catalyst system. Such a polymerization catalyst system comprises an inorganic oxide supported chromium oxide catalyst system and a chromocene compound, wherein the supported catalyst system and chromocene compound are contacted in a polymerization reactor. The resultant, recovered polymer has at least a bimodal, or broad, molecular weight distribution.

This application is a continuation of application Ser. No. 08/202,413,filed Feb. 25, 1994, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to a method for preparing polyolefins having amultimodal molecular weight distribution.

Polyolefins having a multimodal molecular weight distribution (MWD),such as polyethylene, can be made into articles by a variety of methods,including, but not limited to, extrusion molding, thermoforming androtational molding, and have advantages over typical polyolefins lackinga multimodal MWD. Polyolefins having a multimodal MWD process moreeasily, i.e., they can be processed at a faster throughput rate withlower energy requirements, and at the same time such polymers exhibitreduced melt flow perturbations and are preferred because of improvedproperties for applications such as blow molding and/or high strengthfilms. Polymers having a multimodal MWD are generally characterized byhaving a broad MWD, or more that one MWD peak, as reflected by sizeexclusion chromatography (SEC) curves.

There are several known methods of producing polyolefins having amultimodal MWD; however, each method has its own disadvantages.Polyolefins having a multimodal MWD can be made by employing twodistinct and separate catalyst systems in the same reactor, eachproducing a polyolefin having a different MWD; however, catalyst feedrates are usually difficult to control and the catalysts can have adetrimental effect on each other. Polymer particles produced from adual, or even multi-, catalyst system frequently are not uniform insize. Thus, segregation of the polymer during storage and transfer canproduce non-homogeneous products.

A polyolefin having a multimodal MWD can also be made by sequentialpolymerization in two or more separate reactors or blending polymers ofdifferent MWD during processing; however, both of these methods increasecapital cost and problems discussed earlier regarding polymersegregation can occur.

Multimodal MWD polyethylenes can also be obtained directly from a singlereactor polymerization processing the presence of a catalyst systemcomprising two or more catalytic sites, such as, for example,metallocenes, wherein each site has different propagation andtermination rate constants. At certain ratios, and in certainpolymerization processes, even catalysts that have different catalyticsites can produce a monomodal, or narrow, MWD polyolefin. Unfortunately,even under ideal conditions, a dual site catalyst system can havedecreased catalytic activity. While not wishing to be bound by theory,it is hypothesized that a metallocene can bind to, and therefor inhibitthe reactivity of, some of the active chromium oxide catalytic sites.Unfortunately, there are limits to know methods of preparing these verydesirable, multimodal, or broad, molecular weight distribution ormultimodal molecular weight distribution polyolefins.

SUMMARY OF THE INVENTION

It is an object of the present invention to produce polyolefins having amultimodal, or broadened, molecular weight distribution.

It is a further object of the present invention to provide an improvedprocess of making polyolefins having a multimodal, or broadened,molecular weight distribution.

In accordance with the present invention, polyolefins having amultimodal molecular weight distribution at the molecular level areprepared by contacting at least one mono-1-olefin in a polymerizationzone, under polymerization conditions, with dual site supported chromiumpolymerization catalyst systems. Such a polymerization catalyst systemcomprises an inorganic oxide supported chromium oxide catalyst systemand a chromocene compound, wherein the supported catalyst system andchromocene compound are contacted in a polymerization reactor. In apreferred embodiment, the dual site catalyst system consists essentiallyof an inorganic oxide supported chromium oxide catalyst system andchromocene. In another embodiment of the invention, the above-describeddual site catalyst system can be used to polymerize olefins. Theresultant, recovered polymer has at least a bimodal, or broad, molecularweight distribution.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Catalyst Systems

As used in this disclosure, the term "support" refers to a carrier foranother catalytic component. However, by no means, is a supportnecessarily an inert material; it is possible that a support cancontribute to catalytic activity and selectivity.

Any support useful to support chromium catalyst systems can be used.Exemplary catalyst supports include, but are not limited to, inorganicoxides, either alone or in combination, phosphated inorganic oxides, andmixtures thereof. Particularly preferred are supports selected from thegroup consisting of silica, silica-alumina, alumina, fluorided alumina,silated alumina, thoria, aluminophosphate, aluminum phosphate,phosphated silica, phosphated alumina, silica-titania, coprecipitatedsilica/titania, and mixtures thereof, fluorided/silated alumina, as wellas any one or more of these supports which can contain chromium.

The presently most preferred catalyst support, because of greatestpolymerization activity, is aluminophosphate, herein after also referredto as AlPO₄, as disclosed in U.S. Pat. No. 4,364,855 (1982), hereinincorporated by reference. Generally, the P/A1 molar ratio in thealuminophosphate support is within a range of about 0.2 to about 1.0 andpreferably, within a range of 0.4 to 0.9 for best catalyst systemproductivity and activity. Preferably, an aluminophosphate support isactivated prior to use. Activation of the support can occur under anyambient, at a temperature within a range of about 200° C. to about 1000°C., preferably from 400° C. to 650° C. for best catalyst system activityand productivity.

In the description herein, the terms "cogel" and "cogel hydrogel" arearbitrarily used to describe cogellation of at least two catalyst systemcomponents, such as, for example, silica and titania and/oraluminophosphate and chromium. The term "tergel" is used to describe theproduct resulting from gelation together of silica, titania, andchromium. Such terms are disclosed in U.S. Pat. No. 3,887,494 (1975),herein incorporated by reference. "Hydrogel" is defined as a supportcomponent containing water. "Xerogel" is a support component which hasbeen dried and is substantially water-free.

Novel catalyst systems useful in this invention are dual component, ordual-site, inorganic oxide supported chromium catalyst systems. Thefirst component of the catalyst system must be a chromium compound. Thechromium component can be combined with a support component in anymanner known in the art, such as forming a co-precipitated tergel.Alternatively, an organic solution, such as, for example, an alcoholsolution, of a soluble chromium component can be added to a xerogelsupport component. While not wishing to be bound by theory, it isbelieved that use of water in this invention to impregnate a supportwith a chromium compound can cause collapse of the pores in the support,and therefor a decrease in catalyst system productivity. Suitablechromium compounds include, but are not limited to, chromium nitrate,chromium acetate, and chromium trioxide. Alternatively, a solution of ahydrocarbon soluble chromium component, such as tertiary butyl chromate,a diarene chromium compound, bis-cyclopentadienyl chromium(II) orchromium acetylacetonate, can be used to impregnate a xerogel support.

The chromium component can be used in any amount sufficient to providepolymerization catalytic activity. Generally, an amount sufficient togive from about 0.05 to about 5, preferably about 0.5 to about 2 weightpercent chromium, based on the total weight of the chromium and supportafter activation, can be used.

The resulting chromium component on an inorganic oxide support then mustbe subjected to activation, or calcination, in an oxygen-containingambient in any manner conventionally used in the art. Because ofeconomy, the preferred oxygen-containing ambient is air, most preferablydry air to maintain catalyst system integrity for best catalyticactivity and productivity. The activation can be carried out at anelevated temperature for about 1/2 to about 50 hours, preferably about 2to about 10 hours, at a temperature within a range of about 400° toabout 900° C. Under these conditions, at least a substantial portion ofany chromium in a lower valence state is converted to a hexavalent form.

A preferred, second type of supported chromium-oxide catalyst systemuseful in this invention is prepared when the resulting, previouslyactivated, supported catalyst system is cooled and subjected to at leasta partial reduction of the hexavalent chromium to a lower valence state.Preferably, a substantial portion of the chromium will be in a divalentstate after the reduction process in order to achieve betterpolymerization activity and productivity.

The reducing agent most preferably is carbon monoxide due to ease ofuse, availability and safety, based on the safety of other reducingagents. The reducing agent can be employed at temperatures between about300° to about 500° C., although a reducing agent is more often employedat temperatures within a range of about 350° to about 450° C. Thepartial pressure of the reducing gas in the reduction operation can bevaries from sub-atmospheric pressure to relatively high pressures, butthe simplest commercial reducing operation is to utilize a dilutesolution of a pure reducing agent at about atmospheric pressure.Usually, a solution of about 10%, by volume, carbon monoxide in an inertambient, such as, for example, nitrogen and/or argon, can be used. Ifdesired, neat reducing agent, such as, for example, undiluted carbonmonoxide, can be used.

The reduction time can vary from a few minutes to several hours or more.The extent of reduction can be followed by visual inspection of catalystsystem color. The color of the initial oxygen-activated catalyst systemis generally orange, indicating the presence of hexavalent chromium. Thecolor of the reduced catalyst system preferably employed in theinvention is blue, indicating that all or substantially all of theinitial hexavalent chromium has been reduced to lower oxidation states,generally a divalent state.

The course of the reduction action of the air-activated orange catalystsystem with a reducing agent can be determined exactly by pulsetitration. A known amount of reducing agent is added per pulse and theamount of evolved, oxidized reducing agent is measured. When reductionis complete, only reducing agent will be present and the catalyst systemis blue is color. The blue, reduced catalyst system can be titrated withpulses of oxygen or any oxidizing agent, to convert the catalyst systemback to the original orange color. When oxidation is complete, theoxidizing agent will be evident in the off-gas.

After reduction, an inert atmosphere, such as argon or nitrogen, is usedto flush out the reducing agent from the reduced, supported firstcatalyst system component. After the flushing treatment, the firstcatalyst system component is cooled to about room temperature, e.g.about 25° C., and is kept under an inert atmosphere, and preferably awayfrom contact with either a reducing agent or an oxidizing agent.

The second catalyst system component useful in this invention must be amember of the chromocene family of compounds, wherein said chromocene isunsupported. The parent compound of the chromocene family is anorganometallic coordination compound, also called a metallocene. Thechromocene portion of the catalyst system comprises a cyclopentadienyl(Cp) chromium compound, preferably a bis-cyclopentadienyl (Cp₂),chromium compound. Such compounds are considered aromatic and aredepicted by a formula such as (C₅ H₅)₂ Cr wherein a chromium atom is"sandwiched" between two cyclopentadienyl rings. Exemplary chromocenesemployed in accordance with this invention are represented by thegeneral formulae (C₅ R'm)R"s(C₅ R'm)Cr, (C₅ R'm)R"s(C₅ R'm)CrQ, and (C₅R'm)CrQ wherein (C₅ R'm) is cyclopentadienyl or substitutedcyclopentadienyl, each R' can be the same or different and is selectedfrom hydrogen or a hydrocarbyl radical selected from alkyl, alkenyl,aryl, alkylaryl, or arylalkyl radicals having from about 1 to about 20carbon atoms or two adjacent carbon atoms are joined together to form aC₄ -C₆ ring, R" is a C₁ -C₄ alkylene radical, a dialkyl germanium orsilicone or alkyl phosphine amine radical bridging to (C₅ R'm) rings, Qis a hydrocarbon radical such as aryl, alkyl, alkenyl, alkylaryl, orarylalkyl radicals having from about 1to about 20 carbon atoms orhalogen and can be the same or different, s is 0 or 1, and m is 4 when sis 1 and m is 5 when s is 0.

The term "bis-(cyclopentadienyl)chromium(II) compound" includes not onlybis-(cyclopentadienyl)-chromium(II) but substituted derivatives thereofin which the cyclopentadienyl rings contain one or more substituentswhich do not affect the ability of the adsorbed substitutedbis-(cyclopentadienyl)chromium(II) compound to function as an ethylenepolymerization catalyst. The specific bis-(cyclopentadienyl)chromium(II)compound, bis-(cyclopentadienyl)chromium(II), sometimes calledchromocene, has the following postulated structure: ##STR1## Otherexemplary bis-(cyclopentadienyl) compounds include, but are not limitedto, bis-(fluoroenyl)chromium(II) ##STR2## and bis-(indenyl)chromium(II),##STR3## as well as bis-(cyclopentadienyl)chromium(II) compounds havingsubstituted ligands of the formula ##STR4## where one or both R groupsare selected from 1-6 carbon atom alkyl radicals. These materials can bethrough of as a divalent cation (chromium) coordinated by two negativelycharged cyclopentadienyl ligands.

Bis-(cyclopentadienyl)chromium(II) compounds are solids which aresoluble in many organic solvents. Preferred solvents are non-polarliquids used at ambient temperatures. Types of suitable solventsinclude, but are not limited to, alkanes, cycloalkanes, alkenes, andaromatic hydrocarbons. Exemplary solvents include pentane, n-hexane,heptane, decane, cyclohexane, methylcyclohexane, benzene, xylenes, andmixtures of two or more thereof. Preferably, a sufficient quantity of asolution of the chromium component is used to completely wet thealuminophosphate support and fill the porous support structure to insureeven distribution of the chromium compound on the support. Generally,the chromium-containing organic solvent solution used to impregnate thesupport contains from about 0.002 to about 25 weight percentorganochromium compound.

A sufficient volume of the solution of the organochromium compound istaken so as to provide from about 0.01 to about 10, preferably about0.05 to about 5, more preferably from 0.1 to 2 weight percent chromium,based on the weight of the inorganic oxide supported chromium oxide.Contacting the support and organochromium solution can be effected inany conventional way, such as, for example, by slurrying, and can be atany convenient temperature. Generally, ambient temperature is used,although temperatures ranging from about the freezing point of thesolvent to as high as about 300° F. can be employed during thecontacting period. Any pressure can be used, although preferredpressures are those which can maintain the solvent in a liquid phase,for ease of contacting. Contact times from a few seconds to severalhours are adequate. The same stoichiometric amounts of chromium andsupport can be used when the chromium component is added as a separatestream to the reactor and contacted with the support in-situ.

Commonly used polymerization cocatalysts can be used, if desired, butare not necessary. Exemplary cocatalysts include, but are not limitedto, metal alkyl, or organometal, cocatalysts, i.e., alkyl boron and/oralkyl aluminum compounds, which can alter melt flow characteristics(melt index or high load melt index) of the resultant polymer. The term"metal" in organometal is intended to include boron. While not wishingto be bound by theory, it is believed a cocatalyst can act as ascavenger to catalyst system poisons.

If the cocatalyst is a trihydrocarbylboron compound, a trialkyl boroncompound is preferred, wherein the alkyl groups have from about 1 toabout 12 carbon atoms and preferably, from 2 to 5 carbon atoms per alkylgroup. Trialkyl boron compounds, such as, for example, tri-n-butylborane, tripropylborane, and triethylborane (TEB) are preferredcocatalysts because these compounds are effective agents to improvepolymer properties, such as, for example, to reduce melt flow and retardpolymer swelling during polymerization. Other suitable boron compoundsinclude trihydrocarbyl boron compounds broadly; triaryl boron compounds,such as, for example, triphenylborane; boron alkoxides, such as, forexample, B(OC₂ H₅)₃ ; and halogenated alkyl boron compounds, such as,for example, B(C₂ H₅)Cl₂. By far, the most preferred cocatalyst istriethylborane, for the reasons given above.

Also suitable, are aluminum compounds of the formula AlR'nX₃ -n where Xis a hydride or halide, R' is a 1 to 12 carbon atom hydrocarbyl radicaland n is an integer of 1 or 3. Triethylaluminum (TEA) anddiethylaluminum chloride are particularly suitable.

A cocatalyst, when used, can be used in an amount within a range ofabout 1 to about 20 parts per million (ppm), or milligrams per kilogram(mg/kg), based on the mass of the diluent in the reactor. Preferably,the cocatalyst is used in an amount within a range of 1 to 12 mg/kg, forcost effectiveness and best resultant polymer properties. Expressed inother terms, a cocatalyst can be present in an amount so as to give anatom ratio of metal to chromium within a range of about 0.5:1 to about10:1, preferably 2:1 to 8:1.

The cocatalyst can either be premixed with a catalyst system or added asa separate stream to the polymerization zone, the latter beingpreferred.

REACTANTS AND REACTION CONDITIONS

Polymerization can be carried out in any manner known in the art, suchas gas phase, solution or slurry conditions, to effect polymerization. Astirred reactor can be utilized for a batch process, or the reaction canbe carried out continuously in a loop reactor or in a continuous stirredreactor.

A preferred polymerization technique is that which is referred to as aparticle form, or slurry, process wherein the temperature is kept belowthe temperature at which polymer goes into solution. Such polymerizationtechniques are well known in the art and are disclosed, for instance, inNorwood, U.S. Pat. No. 3,248,179, the disclosure of which is herebyincorporated by reference.

The preferred temperature in the particle form process is within a rangeof about 185° to about 230° F. (85° to 110° C.), although higher orlower temperatures can be used. Two preferred polymerization methods forthe slurry process are those employing a loop reactor of the typedisclosed in Norwood and those utilizing a plurality of stirred reactorseither in series, parallel or combinations thereof wherein the reactionconditions are different in the different reactors. For instance, in aseries of reactors a chromium oxide catalyst system can be utilizedeither before or after a reactor utilizing a supported chromocenecatalyst system. In another specific instance, a conventional, supportedchromium oxide catalyst system can be utilized in a reactor in parallelwith a reactor utilizing a supported chromocene catalyst system and theresulting polymerization products combined prior to recovering thepolymer.

The molecular weight of the polymer can be controlled by various meansknown in the art such as adjusting the temperature (higher temperaturegiving lower molecular weight) and introducing, or varying the amountof, hydrogen to alter the molecular weight, or varying the catalystcompounds.

POLYMER CHARACTERISTICS

Polymers produced in accordance with this invention have increaseddensity, broadened molecular weight distribution especially on the lowmolecular weight end, increased MI, and increased HLMI, as compared topolymers prepared in the absence of chromocene. While not wishing to bebound by theory, it is believed that polymers produced in accordancewith this invention are unique in that the polymer chains areintertwined in each polymer particle; each polymer particle can beconsidered "all-inclusive" as to polymer characteristics. This catalystsystem composition most preferably is applicable for use with ethylenepolymerization.

The resultant ethylene polymer will usually have a density within arange of about 0.91 to about 0.975 g/cc, and preferably within a rangeof about 0.945 to about 0.96 g/cc. The polymer melt index (MI) isusually within a range of about 0.015 to about 0.7 g/10 min andpreferably within a range of about 0.02 to about 0.5 g/10 min. Thepolymer high load melt index (HLMI) of the resultant polymer willusually be within a range of about 1 to about 175 g/10 min andpreferably within a range of about 4 to about 70 gg/10 min. The shearratio (HLMI/MI) is usually within a range of about 40 to about 250, andpreferably within a range of about 50 to 200. Polymers withcharacteristics within the given ranges are especially useful forapplications of blow molding and/or film production.

EXAMPLES

The following Examples illustrate various aspects of the invention. Dataare included for each example about the polymerization conditions, aswell as the resultant polymer. All chemical handling, includingreactions, preparation and storage, was performed under a dry, inertatmosphere (usually nitrogen). Unless otherwise indicated, bench scalepolymerizations were completed in a 2 or 2.65 liter autoclave reactor at95° C. using an isobutane (1 or 1.2 liter, respectively) slurry.Approximately 0.08 grams of conventional, supported chromium oxidecatalyst system was charged to the reactor against a counter current ofisobutane. If hydrogen was charged to the reactor, hydrogen addition wasfollowed by isobutane. Isobutane was flushed into the reactor with asmall amount of ethylene. Where applicable, 1-hexane was added, followedby ethylene to bring the total reactor pressure to 550 psig. In somecases, 1-hexene was charged half-way through the isobutane charge.Co-catalysts, if used, were added half-way through the isobutane chargeor with the 1-hexene. Ethylene was fed on demand and polymerizationreaction time usually was about 60 minutes.

Supported chromium oxide (Cr/AlPO₄) catalyst systems were prepared oneof two ways. The first method of preparation comprised contacting dryaluminophosphate, as disclosed in U.S. Pat. No. 4,364,855 (1982), havinga P/Al molar ratio of 0.3, 0.6, or 0.9, with an appropriate amount ofCr(NO₃)₃ ·9H₂ O in methanol using an incipient wetness technique, i.e.,such that the chromium/methanol solution just filled the pores of thesupport. The resulting supported chromium catalyst system compositionwas dried in a vacuum oven at 95° C. for 18 hours; activated by beingfluidized in air at 600° C. for 3 hours; and optionally, reduced incarbon monoxide (CO) at 350° C. for 30 minutes. Catalyst systems werestored under nitrogen.

Alternatively, catalyst systems were prepared as described earlier,except that Cr(NO₃)₃ ·9H₂ O was cogelled with the aluminum andphosphorous compounds. The resulting catalyst systems were dried,activated and stored, as described earlier. Catalyst systems prepared inthis manner are referred to as "cogel" or "cogelled".

Separately prepared supported chromocene catalyst systems were preparedand used according to the following procedure. A 40-70 mg portion ofcalcined aluminophosphate support was placed in a small filter tube andslurried with 2mL hexane. Then, 0.8-1.4 mL of CrCp₂ (Cr(C₅ H₅)₂) inhexane solution (1 mg Cr/mL) was added, and the slurry was stirred.After about 1 minutes, the red-orange CrCp₂ solution became clear andthe support turned black. The supported CrCp₂ slurry was filtered andwashed two times with 3 mL hexane, always taking care not to let thesupported CrCp₂ become dry. The chromocene loading on the support wasabout 2 wt. % in all Runs. The slurry volume was brought to 4 mL withhexane and 0.3 mL of 0.5 weight percent triethylaluminum (TEA), inhexane, was added. The slurry was drawn into a syringe, taken out of thedry box, and charged to a polymerization reactor.

Dual site catalyst systems were prepared in one of two ways. In thefirst method, the polymerization reactor was charged with anabove-described, activated, and optionally CO reduced, Cr/AlPO₄ catalystsystem, followed by a hydrogen charge and one-half of the isobutanecharge. Then, a hexane solution of bis-cyclopentadienyl chromium (CrCp₂)(1mg Cr/mL) was flushed into the reactor with the remaining isobutanecharge, while stirring, to allow the CrCp₂ to contact the supportedchromium oxide catalyst system in the reactor. If any cocatalyst wasused, then a portion of the isobutane charge was saved to carrycocatalyst into the reactor. Addition of 1-hexene was followed byethylene to bring the reactor to 550 psig.

The supports could be fluorided by dissolving the appropriate amount ofammonium bifluoride (also called ammonium hydrogen difluoride, NH₄ F·HF)in methanol and contacting it with the support using incipient wetnesstechnique. The support was then dried in a vacuum oven at 95° C. for 18hours. The fluoride could be added prior to or after the Cr(NO₃)₃ ·9H₂O. The catalyst was then activated as described above.

Alternatively, dual site catalyst systems were prepared by contacting anabove-described, activated, and optionally reduced, Cr/AlPO₄ catalystsystem with a hexane solution of cyclopentadienyl chromium (CrCp₂) undera dry, inert atmosphere, followed by washing with hexane to remove anyexcess cyclopentadienyl ligand.

After polymerization, polyethylene fluff was stabilized, dried at 50° C.under vacuum, and melt blended in a midget Banbury mixer or extruded ona Braebender twin screw extruder. Recovered polymers which had very highor very low high low melt indices (HLMI) were not melt blended.

Polymer density was determined in grams per cubic centimeter (g/cc) on acompression molded sample, cooled at about 15° C. per hour, andconditioned for about 40 hours at room temperature in accordance withASTM D1505 and ASTM D1928, procedure C. High load melt index (HLMI, g/10min) was determined in accordance with ASTM D1238 at 190° C. with a21,600 gram weight. Melt index (MI, g/10 min) was determined inaccordance with ASTM D1238 at 190° C. with a 2,160 gram weight.Environmental Stress Crack Resistance (ESCR, hrs) was determinedaccording to ASTM D1693, Conditions A and/or C. Flexural modulus (FlexMod, Kpsi) was determined in accordance with ASTM D790, 2 inch span, 1mm/sec crosshead speed. Size exclusion chromatography (SEC) analyseswere performed at 140° C. on a Waters, model 150 GPC with a refractiveindex detector. A solution concentration of 0.25 weight percent in1,2,4-trichlorobenzene was found to give reasonable elution times. Smokemeasurements were made as described in U.S. Pat. No. 5,110,214, hereinincorporated by reference.

Polymers (resins) were fractionated in accordance with the followingprocedure. A 6.0 g portion of the resin was dissolved in a solution of75 volume % 1,2,4-trimethylbenzene (TMB) and 25 volume % butylcellosolve(BCS) at 130° C. This solution was gravity fed into a column of veryfine glass beads and cooled to room temperature. Next, BCS was pumpedinto the column and a room temperature solubles (effluent) fraction wastaken as 75% TMB (by volume) was removed from the column. With BCS inthe column, the temperature was raised to 110° C. and another fractionwas collected; 12% TMB and 88% (by volume) BCS was pumped into thecolumn, the temperature was raised again to 130° C. and another fractionwas collected. The column was allowed to equilibriate overnight. Thefraction and collection process was repeated, each time increasing theTMB concentration, until no more polymer could be extracted off thecolumn. The column temperature was raised to 140° C. to remove the lastpolymer fraction. Resins were recovered by precipitating each polymerfraction into acetone which was twice the polymer fraction volume. Theprecipitated resins were filtered and dried at 80° C. in a vacuum oven.Dried resins were analyzed by GPC and ¹³ C NMR.

Solution ¹³ C NMR spectra were collected from a deuteratedtrichlorobenzene solution using either a GE QE200 NMR at 75.5 MHz, or aVarian 500 NMR at 125.7 MHz.

EXAMPLE 1

Data given below in Table 1 compares oxidized and reduced forms ofsupported chromium polymerization catalyst systems wherein comonomer hasor has not been added. In all Runs, the catalyst system had a two (2)weight percent chromium loading, 1.2 liters of isobutane were added,reactor temperature was 95° C. and 35 psi hydrogen was added. All Runswere for 60 minutes, except Run 104, which was for 40 minutes; Run 110,which was for 45 minutes and Run 112, which was for 50 minutes. As canbe seen from the resultant polymer density, a polymer prepared usingonly a CO reduced, supported chromium oxide catalyst system has a lowerdensity and, therefore, is considered to be more efficient inincorporating 1-hexane into the polymer. Alternatively, catalyst systemswhich have not been CO reduced result in polymers which have higherdensity values and greater HLMI values.

                  TABLE 1                                                         ______________________________________                                                                    Product-                                               P/Al,           Catalyst                                                                             ivity,                                                 Molar   1-C.sub.6.sup.=,                                                                      Treat- g pol/ HLMI,  Density,                            Run  Ratio   mL      ment.sup.(a)                                                                         g cat  g/10 min                                                                             g/cc                                ______________________________________                                        101  0.3     0       Ox     2560   3.71   0.959                               102  0.3     0       Red    3980   0.60   0.955                               103  0.3     20      Ox     1760   27.0   0.953                               104  0.3     20      Red    4850   2.27   0.945                               105  0.6     0       Ox     1980   6.62   0.961                               106  0.6     0       Red    4590   2.20   0.958                               107  0.6     20      Ox     1350   80.4   0.957                               108  0.6     20      Red    5880   6.66   0.950                               109  0.9     0       Ox     2000   7.15   0.962                               110  0.9     0       Red    2250   4.06   0.960                               111.sup.(b)                                                                        0.9     20      Ox     1800   154    0.957                               112  0.9     20      Red    4160   14.6   0.950                               ______________________________________                                         .sup.a) Ox = oxidized only; Red = oxidized and CO reduced.                    .sup.b) 2.5 mg/kg triethylaluminum (TEA) was added.                      

EXAMPLE 2

Data for polymerization reactions wherein CrCp₂ was adsorbed onto asupported chromium oxide catalyst system are given in Table 2. The P/Almolar ratios of the supports were varied. In all Runs, reactortemperature was 95° C. and 35 psi hydrogen was added. All Runs were for60 minutes, except Run 201, which was for 48 minutes; Run 210, which wasfor 45 minutes; Run 211, which was for 43 minutes; and Run 212, whichwas for 35 minutes. As can be seen from the data in Table 2, the effectof addition of CrCp₂ onto a supported chromium oxide catalyst system ismore pronounced when a CO reduced chromium oxide catalyst system is usedthan when an oxidized-only supported chromium catalyst system is used.

                                      TABLE 2                                     __________________________________________________________________________       P/Al                                                                          Molar                                                                             CrCp.sub.2                                                                        Catalyst                                                                             Other   MI,  HLMI,                                                                              Den,                                                                             Productivity,                          Run                                                                              Ratio                                                                             wt %                                                                              Treatment.sup.(a)                                                                    Components.sup.(b)                                                                    g/10 min                                                                           g/10 min                                                                           g/cc                                                                             g pol/g cat                            __________________________________________________________________________    201                                                                              0.3 0   Ox     2.8     0.04 4.5  0.958                                                                            3180                                   202                                                                              0.3 0   Red    2.8     0    2.38 0.954                                                                            4050                                   203                                                                              0.3 0.31                                                                              Ox     2.8     0.02 5.17 0.959                                                                            2740                                   204                                                                              0.3 0.28                                                                              Red    2.8     0.86 85.2 0.963                                                                            1400                                   205                                                                              0.6 0   Ox     --      0.03 6.62 0.961                                                                            1980                                   206                                                                              0.6 0   Red    20      0.04 6.66 0.950                                                                            5880                                   207                                                                              0.6 0.29                                                                              Ox     --      0.02 4.97 0.962                                                                            1160                                   208                                                                              0.6 0.26                                                                              Red    20      0.54 64.8 0.958                                                                            2520                                   209                                                                              0.9 0   Ox     --      0.03 7.15 0.962                                                                            2000                                   210                                                                              0.9 0   Red    --      0.01 4.06 0.960                                                                            2250                                   211                                                                              0.9 0.36                                                                              Ox     Method 2.sup.(c)                                                                      7.35 234  0.963                                                                             800                                   212                                                                              0.9 0.35                                                                              Red    --      3.6  167  0.965                                                                            1890                                   __________________________________________________________________________     .sup.a) Ox = oxidized only; Red = oxidized and CO reduced.                    .sup.b) Runs 201-204 had triethylaluminum (TEA) added, given in mg/kg         (ppm); Runs 206 and 208 had 1hexene added, given in mL.                       .sup.c) Refers to the method of preparation of the supported chromium         oxide catalyst system.                                                   

EXAMPLE 3

The data in Table 3 show the effect of the molar ratio of phosphorous toaluminum on catalyst system productivity. In all Runs, 35 psig hydrogenwere added to the reactor and the reaction time was 60 minutes. No1-hexene was added in Runs 301 and 303 and 20 mL 1-hexene was added tothe reactor in Run 302. Polymer produced in Run 303 was blended in aBanbury mixer prior to analysis.

                  TABLE 3                                                         ______________________________________                                              P/Al                      MI,  HLMI,                                          Molar   CrCp.sub.2,                                                                            Productivity,                                                                          g/10 g/10  Den,                               Run   Ratio   Wt %     g pol/g cat                                                                            min  min   g/cc                               ______________________________________                                        301   0.3     0.28     1400     0.86 85.2  0.963                              302   0.6     0.26     2520     0.54 64.8  0.958                              303   0.9     0.19     2680     0.18 16    0.961                              ______________________________________                                    

EXAMPLE 4

The data in Example 4 show the effect of fluoriding the support. In allRuns, 35 psig hydrogen and 20 mL of 1-hexene were added to the reactor.Run 406 had 2.8 mg/kg (ppm) TEA added to the reactor. Run 402 wasblended in a Banbury mixer; Runs 404, 405 and 406 were blended in aBraebender extruder.

                  TABLE 4                                                         ______________________________________                                             P/Al                  Produc- MI,  HLMI,                                      Molar   F,.sup.(a)                                                                            CrCp.sub.2,                                                                         tivity, g/10 g/10  Den,                            Run  Ratio   wt %    wt %  g pol/g cat                                                                           min  min   g/cc                            ______________________________________                                        401  0.3     --      0.26  1490    0.13 19    --                              402  0.3     1       0.31  2270    0.29 55    0.958                           403  0.3     2       0.30  1790    0.18 26    --                              404  0.6     --      0.26  2380    0.20 22    0.952                           405  0.6     1       0.26  1160    0.22 24    0.955                           406.sup.(b)                                                                        0.6     1       0.25  2120    0.97 87    0.959                           ______________________________________                                         .sup.a) Based on total weight of support only.                                .sup.b) TEA, 2.8 mg/kg, added to reactor.                                

EXAMPLE 5

The data in Table 5 show the effect of using a cogel as the support forthe dual site catalyst system and that the productivity is significantlylowered. In all Runs, reactor temperature was 95° C. and 35 psighydrogen and 20 mL 1-hexene were added to the reactor. All chromiumoxide catalyst systems were CO reduced prior to the addition of CrCp₂.Run time in all Runs was 60 minutes, except Run 506 which was for 40minutes. Polymer produced in Run 501 was blended in Banbury mixer. Runs502, 503, 504, and 506 used a cogel catalyst system support.

                  TABLE 5                                                         ______________________________________                                              P/Al                      MI,   HLMI,                                         Molar   CrCp.sub.2,                                                                            Productivity,                                                                          g/10  g/10  Den,                              Run   Ratio   Wt %     g pol/g cat                                                                            min   min   g/cc                              ______________________________________                                        501   0.6     0.26     2520     0.54  65    0.958                             502   0.6     0.09     2080     0.04  7.5   --                                503   0.6     0.19     780      --    155   --                                504   0.6     0.26     800      18    807   --                                505   0.3     0.26     1490     0.13  19    --                                506   0.3     0.21     460      --    95    --                                ______________________________________                                    

EXAMPLE 6

The data in Table 6 show the amount of smoke generated from both the"multicomponent" resins and resins produced from the reduced Cr/AlPO₄catalyst alone. Runs 603 and 606 had one (1) weight percent fluorideadded to the support. In all Runs, 35 psig hydrogen were added to thereactor. In all Runs, 20 mL 1-hexene was added to the reactor. Run 601had 10 mL 1-hexene and 2 ppm TEB added to the reactor. All run timeswere 60 minutes, except Run 602, which was for 40 minutes. Runs 601 and604 were blended in a Banbury mixer, Run 605 was blended in a Braebenderextruder and Run 606 was blended in an extruder. The data show that Runsusing a dual catalyst system can produce polymers with lower smokevalues than polymers produced using a CO reduced Cr/AlPO₄ catalystsystem alone.

                  TABLE 6                                                         ______________________________________                                             P/Al            Produc- MI,  HLMI,                                            Molar   CrCp.sub.2,                                                                           tivity, g/10 g/10  Den, Smoke,                           Run  Ratio   Wt %    g pol/g cat                                                                           min  min   g/cc Mg/m.sup.3                       ______________________________________                                        601  0.6     --      4340    0.14 25    0.958                                                                              >2100                            602  0.6     --      2570    0.11 19    0.946                                                                              >2100                            603  0.6     --      3730    0.08 12    0.948                                                                              >2100                            604  0.6     0.26    2520    0.54 65    0.958                                                                              1090                             605  0.6     0.26    2380    0.20 22    0.952                                                                              1600                             606  0.6     0.25    2120    0.97 87    0.959                                                                              980                              ______________________________________                                    

EXAMPLE 7

The data in Tables 7 and 8 show the chromocene is an active component ona reduced Cr/AlPO₄ catalyst. All Runs were at 95° C. and had 35 psighydrogen added to the reactor. Aluminophosphate supports in Runs 701 and702 were calcined at 450° C. and in Run 703 at 600° C. Chromocene isknown to produce methyl branches on the polymer back bone. The amount ofmethyl groups found as well as the amount of comonomer in resinsproduced from only chromocene adsorbed onto various aluminophosphatesupports are listed below in Table 7.

                  TABLE 7                                                         ______________________________________                                                                         #CH.sub.3                                                                             #C.sub.4 H.sub.9                                 1-hexene      Density                                                                              Branches/                                                                             Branches/                            Run  P/Al   mL       MI   g/mL   1000 C  1000 C                               ______________________________________                                        701  0.3    40       114  0.9669 2.1     0                                    702  0.6    30       65   --     2.7     0                                    703  0.9    30       141  --     1.7     0                                    ______________________________________                                    

Table 8a gives the results of two polymers which were fractionated todetermine the location of the short chain branches (SCBs) in thepolymer. All runs were at 95° C. using 35 psi hydrogen and thecorresponding amount of 1-hexene. The number of branches were determinedby ¹³ C NMR. Polymer produced in Run 801 was extruded with a Maxwellmini-extruder prior to analysis. Polymer in Run 802 was mixed in aBanbury mixer prior to analysis.

                  TABLE 8a                                                        ______________________________________                                               P/Al       CrCp.sub.2,                                                                           MI,    HLMI,  Density                               Run    Molar Ratio                                                                              Wt %    g/10 min                                                                             g/10 min                                                                             g/mL                                  ______________________________________                                        801    0.6        0       0.07   11.67  0.949                                 802    0.6        0.3     0.54   64.77  0.959                                 ______________________________________                                    

Table 8b contains data pertaining to the fractionation of amulticomponent resin made with chromocene adsorbed onto a reducedCr/AlPO₄ catalyst and the reduced Cr/AlPO₄ catalyst by itself. Resin 801and 802 were prepared as described in Table 8a. The amount of methylbranching can be found under the mol % C₃ ; it can be viewed that amethyl branch is a result of a propylene monomer unit. The amount ofethyl branching can be found under the mol % C₄ ; the amount of butylbranching can be found under the mol % C₆. The data in Table 8b showthat chromocene is an active component of the catalyst system, based onthe presence of methyl branches. The data also show that the CO-reducedportion of the catalyst system is active, based on the presence of butylbranches (as a result of incorporating 1-hexene). Furthermore, while notwishing to be bound by theory, it is believed that chromocene is theactive component in production of the low molecular weight portion ofthe material.

                  TABLE 8b                                                        ______________________________________                                                                            mol  mol  mol                                                                 %    %    %                               Run    No.     M.sub.w-3 × 10                                                                    IB   wt. % C.sub.6                                                                            C.sub.4                                                                            C.sub.3                         ______________________________________                                        801    parent  225.84    1.650                                                                              100   0.305                                                                              0.07 0.02                            (fraction)                                                                           1       1.34      0.308                                                                              1.46  0.614                                                                              0.27 0.01                                   2       2.46      0.373                                                                              2.98  0.480                                                                              0.137                                                                              0.01                                   3       4.69      0.474                                                                              8.04  0.202                                                                              0.061                                                                              0.02                                   4       8.11      0.313                                                                              3.94  0.324                                                                              0.072                                                                              0.01                                   5       74.99     1.236                                                                              62.47 0.376                                                                              0.034                                                                              0.02                                   6       745.28    0.793                                                                              15.55 0.220                                                                              0.026                                                                              0.01                                   7       2535.43   0.913                                                                              3.87  a    a    a                               802    parent  201.90    1.621                                                                              100   0.163                                                                              0.027                                                                              0.13                            (fraction)                                                                           1       1.34      0.316                                                                              1.40  0.518                                                                              0.233                                                                              0.45                                   2       2.22      0.363                                                                              2.82  0.320                                                                              0.104                                                                              0.28                                   3       4.59      0.486                                                                              10.62 0.109                                                                              0.041                                                                              0.124                                  4       7.67      0.329                                                                              5.62  0.164                                                                              0.025                                                                              0.132                                  5       64.52     1.105                                                                              66.31 0.183                                                                              0.024                                                                              0.145                                  6       1041.10   0.894                                                                              10.31 0.166                                                                              0.012                                                                              0                                      7       3074.00   0.899                                                                              1.80  a    a    a                               ______________________________________                                         a) These samples were only sparingly soluble, therefore a good NMR            spectrum could not be obtained.                                          

While this invention has been described in detail for the purpose ofillustration, it is not to be construed as limited thereby but isintended to cover all changes and modifications within the spirit andscope thereof.

That which is claimed is:
 1. A process for producing polyolefins havinga multimodal molecular weight distribution comprising:1) polymerizingunder polymerization conditions in a polymerization zone at least oneolefin in the presence of a dual site catalyst system comprising:a) achromium oxide catalyst system comprising chromium oxide supported on aninorganic oxide support prepared from an inorganic support impregnatedwith a chromium compound and activated in the presence of oxygen and b)a chromocene compound and 2) recovering of polymer.
 2. A processaccording to claim 1 further comprising a metal alkyl cocatalyst.
 3. Aprocess according to claim 1 wherein said olefin is selected from thegroup consisting of ethylene, propylene, 1-butene 1-pentene, 1-hexene,1-octene, and mixtures thereof.
 4. A process according to claim 3wherein said olefin is ethylene.
 5. A process according to claim 1wherein said olefin includes ethylene and from about 0.5 to about 20mole percent of a monomer selected from the group consisting ofpropylene, 1-butene, 1-pentene, 1-hexene, 1-octene, and mixturesthereof.
 6. A process according to claim 1 wherein said polymerizationconditions comprise slurry phase polymerization conditions.
 7. A processaccording to claim 3 wherein said slurry phase polymerization conditionscomprise a temperature within a range of about 80° to about 110° C. anda pressure of about 250 to about 700 psia, for a time within a range ofabout 1 minute to about 6 hours.
 8. A process according to claim 1wherein said chromocene compound is a bis-(cyclopentadienyl)chromium(II) compound.
 9. A process according to claim 1 wherein saidinorganic oxide support is aluminophosphate with a phosphorus toaluminum molar ratio within a range of about 0.2 to about 1.0.
 10. Aprocess according to claim 2 wherein said organometal cocatalyst isselected from the group consisting of alkyl boron compounds, alkylaluminum compounds, and mixtures thereof.
 11. A process according toclaim 10 wherein said organometal cocatalyst is an alkyl boron compound.12. A process according to claim 10 wherein said organometal cocatalystis an alkyl aluminum compound.
 13. A process according to claim 1wherein after activation, the chromium oxide catalyst system issubjected to at least a partial reduction.